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~7he equation used for the accounting of <br />• ~. r--• <br />W = I -Eta -DS <br />Where W is the deep percolation (subsurface runoff), I is infiltration, Eta is actual <br />evapotranspiration and S is the change in soil moisture storage. The maximum <br />volume per unit area that can be stored in the root zone for prolonged time is the <br />available water capacity. At any particular time, the water available in the root <br />zone cannot exceed the available wafer capacity. In any particular month, if The <br />sum of infiltration and stored water (available wafer) exceed the evapotranspiration <br />requirement, then deep percolation occurs. If the sum of infiltration and stored <br />water in a particular month are less Than the evapotranspiration requirement, then <br />evapotranspiration is limited by this sum. <br />An average effective rooting depth of 36 inches was used together with an <br />available water capacity of 0.07 in/in to obtain tiviches for the available water <br />capacity of The root zone on the Edna Mine. These values were estimated from soils <br />information, primarily on the Moffat Area, provided through Pittsburg 8~ Midway by <br />• the soils investigation. The subsurface water balance computations are shown in <br />Table 2.5-24 where ii is indicated shot, on an average annual basis, 16.68 inches of <br />water infilirote the soil, 14.73 inches are consumptively used, and 1.95 inches <br />becomes subsurface runoff. <br />The computations in Table 2.5-24 actually begin for November so that iT can <br />be assumed that the available water is zero at the beginning of The month and the <br />deficit in soil-water storage is the maximum. Because 0.40 inches infiltrates in <br />November and evapotranspiration is zero, the entire infiltrated quantity goes toward <br />making up the deficit. Al~+de,~~tm1Sf},.,gcr~F,~;;rgcl~k.Rl~errsst;~xnplene9'y <br />mode up. Through the winter months, the avoilable water remains unchonged so shot <br />0.40 inches are available at the beginning of April and the deficit is 2.12 inches. <br />During April, 4.04 inches of infiltration occurs. This is sufficient to make up the <br />remainder of the deficit, satisfy the evapotranspiration demand of 1.52 inches and <br />'~lu~~ r~f~'zx~ad~ A similar situation <br />occurs in May. In June, however, even the full compliment of stored water and the <br />June infiltration ore not sufficient to satisfy the evapotranspiration demand of <br />. inches. Therefore, the actual evapotranspiration is limited to the sum of the water <br />available in storage and the June infiltration. The available water in storage is <br />2.5-86 <br />